Loudspeaker systems capable of generating large power outputs have been developed using several loud speakers, see for example the loudspeaker systems described in U.S. Pat. No. 3,582,553 to Bose or U.S. Pat. No. 4,031,318 to Pitre. When large output power is to be achieved with small enclosures, radiation from the back of the cones of small speakers often becomes a source of distortion; hence, absorbent materials are employed to reduce this effect. The absorbent material, however, alters the frequency response of the speaker system, usually causing an excessive amount of droop at low frequencies.
Techniques have been proposed to improve the low frequency response and efficiency of a small speaker system. In one technique, such as described in the U.S. Pat. No. 3,952,159 to Schott, a bass-reflex speaker enclosure is described with a ducted port to reduce low frequency distortion. In the U.S. Pat. No. 1,988,250 to Olson and U.S. Pat. No. 3,780,824 to Prince, high compliance passive radiators are employed in a fluid-tight enclosure to improve the low frequency response of the speaker system. A pair of symmetrically disposed passive radiators are shown in Olson for better appearance.